Image forming apparatus forming images by using an electrophotographic image forming method (hereinafter also referred to simply as “electrophotographic apparatus”) has been often used, for example, in copying machines, printers, or facsimile units. In the electrophotographic apparatus, images are formed by way of the following electrophotographic process. At first, the surface of an electrophotographic photoreceptor provided to the apparatus (hereinafter also referred to simply as “photoreceptor”) is charged to a predetermined potential uniformly. Exposure is applied to the surface of the charged photoreceptor in accordance with image information to form electrostatic latent images. The formed electrostatic latent images are developed by a developer containing a toner and the like to form toner images as visible images. The formed toner images are transferred from the surface of the photoreceptor onto a body to be transferred and fixed to form images.
The electrophotographic photoreceptor includes a conductive substrate comprising a conductive material and a photosensitive layer disposed on the conductive substrate. As the material for constituting the photosensitive layer of the photoreceptor, inorganic photoconductive materials such as selenium, cadmium sulfide or zinc oxide have been known so far. While inorganic photoreceptors using such inorganic photoconductive materials have various advantageous that they can be charged to an appropriate potential in a dark place, release less charges in the dark place and can release charges rapidly by irradiation of light, they also have various drawbacks. For example, selenium type photoreceptors using selenium require difficult manufacturing conditions and need high manufacturing cost. Further, since they are sensitive to thermal or mechanical impacts, a care has to be taken during handling. A cadmium sulfide type photoreceptor using cadmium sulfide and a zinc oxide type photoreceptor using zinc oxide can not obtain a stable sensitivity under a high humidity circumstance and, in a case where a dye is added as a sensitizer, since the dye is degraded under charging by corona charging upon charging the surface of the photoreceptor or optically discolored by exposure, stable characteristics can not be provided for a long time.
As described above, since the inorganic photoreceptors have many drawbacks, organic photoreceptors using organic photoconductive materials have been proposed instead of the inorganic photoreceptors as the photoreceptor. For example, photoreceptors using various organic photoconductive polymers including polyvinyl carbazole are present. However, while the photoreceptors using the polymers are excellent in view of the film forming property and decrease of weight of the photosensitive layer compared with the inorganic photoreceptors using the inorganic photoconductive materials described above, they have a drawback of being poor in view of the sensitivity, the durability and stability to the change of the circumstance.
Various research and development have been conducted for overcoming such drawbacks, and a function separated photoreceptor has been proposed in which a charge generation function and a charge transport function as the photoconductive function due to the organic photoconductive polymer in the organic photoreceptor are shared on separate materials respectively. The function separated photoreceptor includes a layered type and a single layer type. In the layered type function separated photoreceptor, a layered type photosensitive layer formed by stacking a charge generating layer containing a charge generating substance responsible for the charge generation function and a charge transporting layer containing a charge transporting substance responsible for the charge transport function is provided. In the single layer type function separated photoreceptor, a photosensitive layer of a single layer type formed by dispersing a charge generating substance and a charge transporting substance in the identical layer is provided.
In the function separated photoreceptor described above, a selection range for the material constituting the photosensitive layer is wide and a photoreceptor of high performance can be provided by combining materials so as to optimize electrophotographic characteristic such as charging characteristic, sensitivity, residual potential characteristic, repetitive characteristic, and printing resistance. Further, since the photosensitive layer can be formed by coating, an inexpensive photoreceptor having extremely high productivity can be provided.
Further, in the function separated photoreceptor, a charge generating substance absorbs light with which the photoreceptor is irradiated to generate charges and the charges are injected to the charge transporting substance and transported to the surface of the photoreceptor to thereby eliminate the surface charges on the photoreceptor at a portion irradiated with light. As described above, since the light with which the photoreceptor is irradiated is absorbed in the charge generating substance, the light sensitive wavelength region of the photoreceptor can optionally be controlled by properly selecting the charge generating substance.
In recent years, for obtaining images of higher quality, storing inputted image information or optionally editing the same, digitalization of image information has been popularized rapidly. While electrophotographic apparatus forming images by using digitalized image information have been restricted so far to laser printers, LED (Light Emitting Diode) printers as output equipments for word processors and personal computers, as well as to some color laser copying machines, digitalization has also been proceeded in the field of usual copying machines in which images were formed predominantly by using analog image information.
In digitalized electrophotographic apparatus, exposure to the surface of the photoreceptor is conducted as described below. For example, in a case of forming images by directly using digital image information prepared by a computer, digital electric signals as image information outputted from the computer are converted into optical signals and the surface of the photoreceptor is irradiated with light corresponding to the optical signals to apply exposure in accordance with the image information to the surface of the photoreceptor. Further, in a case of forming images by using image information read from original document images as in the case of copying machines, image information for the document images are read as optical signals, converted into digital electric signals and then again converted into optical signals, and the surface of the photoreceptor is irradiated with light corresponding to the optical signals to thereby apply exposure to the surface of the photoreceptor in accordance with the image information.
In the digital electrophotographic apparatus, a laser light or LED light has been used mainly for the light as light corresponding to the optical signals which are digital image information with which light the surface of the photoreceptor is irradiated. Among them, a light used most frequently is a near infrared light at a wavelength of 780 nm or a light in a long wavelength region such as a red light at a wavelength of 660 nm. Accordingly, what is demanded at first for the photoreceptor used for the digital electrophotographic apparatus is that it has a sensitivity to the light in such long wavelength region.
As described above, the light sensitive wavelength region of the photoreceptor can be optionally selected by properly selecting the charge generating substance. As the charge generating substance showing sensitivity to a light in the long wavelength region such as the near infrared light or red light described above, versatile materials have been studied so far. Among them, since phthalocyanine compounds are synthesized relatively easily and most of them show the sensitivity to the long wavelength light, they have been studied generally and put to practical use.
For example, there have been proposed a photoreceptor using oxotitanium phthalocyanine (refer to Japanese Examined Patent Publication JP-B2 5-55860 (1993)), a photoreceptor using a β-type indium phthalocyanine (refer to Japanese Unexamined Patent Publication JP-A 59-155851 (1984)), a photoreceptor using X-type non-metal phthalocyanine (refer to Japanese Unexamined Patent Publication JP-A 2-233769 (1990)) and a photoreceptor using oxovanadium phthalocyanine (refer to Japanese Unexamined Patent Publication JP-A 61-28557 (1986)).
Recently, it has been found that oxotitanium phthalocyanine having specified crystal forms shows particularly high sensitivity to a long wavelength region and photoreceptors using them have been proposed. They are photoreceptor using, for example, oxotitanium phthalocyanine having a crystal form showing a maximum diffraction peak at the Bragg angle of 2θ (error: 2θ±0.2°) of 27.3° and showing a diffraction peak at 7.4°, 9.7° and 24.2° (refer to Japanese Examined Patent Publication JP-B2 7-91486 (1995)), oxotitanium phthalocyanine having a crystal form showing main diffraction peaks at the Bragg angle 2θ (error: 2θ±0.2°) of 9.5°, 9.7°, 11.7°, 15.0°, 23.5°, 24.1°, and 27.3° (refer to Japanese Examined Patent Publication JP-B2 2700859) or, oxotitanium phthalocyanine having a crystal form showing main diffraction peaks at the Bragg angle 2θ (error: 2θ±0.2°) of 9.0°, 14.2°, 23.9°, and 27.1° (refer to Japanese Unexamined Patent Publication JP-A 3-128973 (1991)) in the X-ray diffraction spectrum.
Further, it has been known that a photoreceptor using oxotitanium phthalocyanine having a crystal form showing, in view of X-ray diffraction spectrum, main diffraction peaks at the Bragg angle 2θ (error: 2θ±0.2°) of 7.3°, 9.4°, 9.6°, 11.6°, 13.3°, 17.9°, 24.1° and 27.2° in which a bundle of diffraction peaks formed by overlap of a diffraction peak at 9.4° and a diffraction peak at 9.6° shows the maximum intensity and a diffraction peak at 27.2° shows the intensity next to the highest has a particularly high sensitivity to a light in a long wavelength region light and has good stability of characteristics during repetitive use (refer to Japanese Unexamined Patent Publication JP-A 2000-129155).
Further, photoreceptors of using two or more kinds of phthalocyanine compounds have also been proposed. They are, for example, a photoreceptor using a phthalocyanine composition containing oxotitanium phthalocyanine and non-metal phthalocyanine and having a diffraction peak at Bragg angles 2θ (error: 2θ±0.2°) of 7.0°, 9.0°, 14.1°, 18.0°, 23.7°, and 27.3° in the X-ray diffraction spectrum to CuKα characteristic X-rays (wavelength: 1.541 Å) (refer to Japanese Unexamined Patent Publication JP-A 2000-313819), and a photoreceptor using mixed crystals of phthalocyanine comprising oxotitanium phthalocyanine and gallium halide phthalocyanine or indium halide phthalocyanine (refer to Japanese Unexamined Patent Publication JP-A 4-372663 (1992)).
On the other hand, for the electrophotographic apparatus, decrease of size and increase in the image forming speed have also been demanded. In the electrophotographic apparatus used for example in copying machines, printers and facsimile units, a photoreceptor having a photosensitive layer provided on the outer circumferential surface of a cylindrical or circular columnar conductive substrate has been used generally and it is necessary to decrease the diameter of the photoreceptor in order to reduce the size of the electrophotographic apparatus. However, in the photoreceptor of a small diameter, since the distance from the exposure position to the developing position is short, when an electrophotographic process is conducted at high speed in order to increase the image forming speed, the time from exposure to the development is shortened to result in the following problems. For example, in a case of using a photoreceptor of low responsivity, that is, a photoreceptor of low decaying speed for the surface potential after exposure, development is conducted in a state where the surface potential at a portion to be erased by the exposure has not yet been decayed sufficiently. Therefore, in a case of normal development, a phenomenon referred to as background contamination occurs in which a toner is deposited to a portion as a white background images and, in a case of reversal development, the image density is lowered. Accordingly, for compatibilizing decrease of the size and increase in the image forming speed of the electrophotographic apparatus, a photoreceptor of high responsivity is demanded.
Since phthalocyanine compounds used in the photoreceptors described above in JP-B2 7-91486, JP-B2 2700859, JP-A 3-128973 and JP-A 2000-129155, JP-B2 5-55860, and JP-A 59-155851, JP-A 61-28557, JP-A 2-233769, JP-A 2000-313819 and JP-A4-372663 have high charge generation ability and high charge injection efficiency, the photoreceptors described in the publications have responsivity to some extent. However, no sufficient responsivity has yet been obtained in the photoreceptors described above since the combination between the phthalocyanine compound as the charge generating substance and the charge transporting substance is not appropriate. Particularly, no sufficient responsivity can be obtained under a circumstance where temperature is low and the humidity is low (hereinafter such circumstance is referred to as “low temperature, low humidity circumstance”), and background contamination or lowering of image density occurs in a case of decreasing the diameter of the photoreceptors and using them in a high speed electrophotographic process. Accordingly, in a case of using the photoreceptors described above while decreasing the diameter, it is necessary to suppress the speed of the electrophotographic process, so that the image forming speed of the electrophotographic apparatus cannot be increased. That is, in a case of using the photoreceptors, it is impossible to compatibilize the decrease of the size and the increase in the image forming speed of the electrophotographic apparatus.